In addition to sending voice and/or data over the telephone lines, a telecommunication service provider may use the set of same wires to power devices that are connected to the signal lines at remote locations. This practice, generally referred to as ‘loop’ or ‘span’-powering, permits the phone company to power equipment located up to several miles away from power source (typically installed in the central office (CO)), and is used extensively in HDSL, HDSL2, HDSL4, DDS, ISDN and T1 network applications.
However, as the distance between the power source and the remotely powered device increases, and it becomes necessary to increase the magnitude of the source voltage to compensate for the additional voltage drop associated with the span resistance, a potential safety hazard (to maintenance personnel, for example) exists once the source voltage is increased to a relatively high value. If an individual inadvertently comes in contact with a powered line, that person becomes a current path to ground. Because the ground fault current through the human body is typically considerably lower than the load current, the high voltage power source will experience only an imperceptible increase in power; however, the ground fault current can be fatal.
While this potential hazard may be avoided by de-energizing the high voltage source, doing so is undesirable, since it would cause a service outage to the customer. To remedy this problem, the telecommunication industry has established voltage and current limits for allowing safe access to a powered span. In order to comply with these standards, yet still provide a relatively high voltage to the remote device, it is common practice to employ a bipolar power source, to reduce the absolute value of a line to ground voltage, yet realizing a higher differential voltage across the powered span.
This approach is diagrammatically illustrated in FIG. 1, which shows the output stage of a typical bipolar power converter (such as that contained in a central office) having an output transformer winding 10 with a first polarity (+) terminal 11 coupled through a diode 21 over a line 31 to one end 41 of a piece of telecommunication equipment, shown as a resistive load 40, and a second polarity (−) terminal 12 coupled through a diode 22 over a line 32 to a second end 42 of the load 40. Output transformer winding 10 has a (center) tap 15 thereof coupled to a ground terminal GND. On the positive side, a storage capacitor 51 is coupled between the cathode of diode 21 and GND; on the negative side, a storage capacitor 52 is coupled between the anode of diode 22 and GND. With this differential polarity configuration the output voltage of the power converter (e.g., +/−140 VDC) can be effectively doubled across the load (e.g., to a value of +280 VDC). Under normal operating conditions, current flows only through the circuit containing the load 40, not to ground.
FIG. 2 shows a modification of the circuit of FIG. 1 for the circumstance that a fault (such as a person—shown as a resistor 61) is placed between the positive (+) output line 31 and ground. In this event, a ‘ground fault’ current flows from the positive (+) output line 31 through resistor 61 to ground. A complementary situation arises when a fault is formed between the negative (−) line 32 and ground. In either event, both immediate detection and compensation for the high voltage and associated ground current condition are required. Conventional proposals to address this problem are often complex and have included the use of Hail effect devices and/or coupled inductors for monitoring current imbalance on the output lines. Other methods such as described in U.S. Pat. No. 5,774,316, generate a separate ground-referenced voltage through which fault current is measured.